Previously, the applicant proposed a screw tightening apparatus provided with an automatic conduction interrupting device. In this automatic conduction interrupting device, when an excessive load current flows in the electric power driver due to an outside torque, the excessive load current is electrically detected, so that conductivity to the motor is interrupted, and, at the same time, the armature of the motor is short-circuited, and electrical braking is applied to the motor, thus instantaneously stopping the rotation of the motor (see Patent Literature 1). More specifically, in the screw tightening apparatus described in Patent Literature 1, a switching circuit is provided in the electric power driver which has a driver shaft rotationally driven by the motor; and in case that a load current, which flows when the motor is driven, changes to an excessive load current of or greater than a prescribed value, then such an load current is detected, and the supply of motor drive current is interrupted, so that, after a certain time interval has elapsed, the supply of motor drive current is restored, and with which also a switching mechanism is provided in the motor armature circuit, and when the switching circuit acts to interrupt the supply of motor drive current, the motor armature circuit is short-circuited, electrical braking is thereby applied to the motor, and thus the rotation the motor is instantaneously stopped.
Conventionally, moreover, DC motors have been commonly used as drive motors for electric power driver. Today, use of brushless motors as DC motors is proposed and practically implemented for the reasons that brushless motors exhibit superior characteristics in terms of non-contact configuration, noise prevention, high-torque miniaturization, high-speed turning, and long life, and that they have the advantage of being maintenance-free.
The above-described brushless motor, however, unlike the case of a conventional DC motor, when drive-controlling a three-phase DC brushless motor, for example, a drive circuit for generating a turning magnetic field is required. Such a drive circuit can be comprised of magnetic sensors (for which Hall elements are generally used) for detecting the position of the magnetic poles of the magnet rotor, drive coils (three-phase coils) magnetized so as to impart a turning force in a certain direction in correspondence with the positions of the rotor magnetic poles, and a dedicated IC circuit (inverter circuit comprising a three-phase full-wave bridge configuration) for controlling the energizing of those magnetic sensors and drive coils.
In view of the above, the applicant has proposed a drive switch system for an electric (power) rotary tool, as an electric (power) rotary tool that utilizes the brushless motor described above, which, according to the nature of the work being done, can selectively effect drive control based on a desired push operation system or lever operation system (see Patent Literature 2).
More specifically, the electric (power) rotary tool drive switch system described in Patent Literature 2 is configured such that (1) the electric (power) rotary tool is comprised of: an electric motor, a rotary bit, such as a driver bit for performing such work as tightening a screw, coupled to the output shaft of the electric motor, a drive switch for driving the electric motor and starting the above-described work by the rotary bit, a torque detection means for detecting the load torque developed in the rotary bit associated with the completion of the work, and a torque setting automatic stopping means for stopping the rotary bit when the load torque reaches a preset torque value; and in this rotary tool, (2) a push operation type drive switch for effecting ON action by bringing the rotary bit up against a screw or other work and causing a pushing-pressure displacement, and (3) a lever operation type drive switch for effecting ON action by pushing-pressure displacing a switch lever provided in the grip of the electric (power) rotary tool, are respectively provided in combination of magnets and magnetic sensors; and (4) the magnetic sensors are respectively connected to the conducting circuit for the electric motor, and one or other of the operation systems is selected; and, by the magnetic sensing action of the magnetic sensors of the selected operation system, the conducting circuit is subjected to an ON action, so that the electric motor is started.
Accordingly, in the electric (power) rotary tool drive switch system configured in the manner described above, when a brushless motor is used as its electric motor, it is possible to accommodate and deploy everything, including the drive switches and a drive control circuit of the motor, compactly inside the grip casing of the electric (power) rotary tool, and long maintenance-free life is achieved by implementing thoroughgoing non-contact and maintenance-free configuration, thus simplifying the way the tool is used.
The applicant further proposed a torque control system for an electric (power) rotary tool. In this torque control system, constant torque tightening work for screws or the like can always be achieved properly and efficiently, and overall downsizing and improved torque control precision enhancement of the tool is easily accomplished (see Patent Literature 3).
More specifically, in the torque control system for an electric (power) rotary tool described in Patent Literature 3, (1) a grip housing an electric motor therein is provided, a follower shaft is coupled, as a work shaft, through a speed reduction mechanism to the output shaft of the electric motor, a clutch mechanism is provided that comprises a cam engagement unit for operating so as to break the coupling between the output shaft and the follower shaft when a load torque of or greater than a prescribed value acts on the follower shaft, a torque setting in which the clutch action point of the clutch mechanism is regulated as a torque setting value is provided, and a torque detection mechanism for detecting the clutch action of the clutch mechanism and simultaneously effecting drive stop-control on the electric motor is provided; and (2) the above-described torque detection mechanism is set so that, when the cam engagement in the cam engagement unit of the clutch mechanism is completely released and the clutch action is finished, the electric motor is subjected to a drive stop-control simultaneously with such detection action.
In this torque control system for an electric (power) rotary tool, the torque detection mechanism is composed of a combination of magnets and magnetic sensors; and when the cam engagement is completely released, a detection action is effected. Accordingly, it is possible to eliminate occurrences of switch contact sparking and wear and also damages imparted to the peripheral electronic equipment and electronic circuitry as occurred in torque detection mechanisms that use a conventional mechanical switch mechanism; and it is further possible to make the parts smaller in size and have a longer life and to make the overall electric power tool size compact.    Patent Literature 1: Japanese Patent Application Publication (Kokoku) No. 557-43389/1982    Patent Literature 2: Japanese Patent Application Laid-Open (Kokai) No. 2002-254346    Patent Literature 3: Japanese Patent Application Laid-Open (Kokai) No. 2002-321166